Recently, because of its high storage capacity, speedy data transfer rates and the like, a holographic ROM system is wining the widespread application to the field of data storage devices.
Referring to FIG. 1, there is shown a diagram of a conventional holographic ROM system. As shown in FIG. 1, the holographic ROM system includes a holographic disk 200, a memory medium, which stores angle-multiplexed holograms (or holographic data representations) and which is removably mounted on a spindle motor 210; a pickup module 100 for optically reading the holographic data representations from the holographic disk 200 and then producing electrical signals in response to the optically read holographic data representations; a signal processing unit 150 for processing the electrical signals from the pickup module 100; and a control unit 300 for controlling the pickup module 100 and the spindle motor 210.
The pickup module 100 is provided with a laser source 102 for generating a reference laser beam; a double-sided reflecting section 103 for reflecting the laser beam; a reflecting section 104 for reflecting the laser beam from the double-sided reflecting section 103; an aperture plate 105 for filtering an output radiation diffracted from the holographic disk 200 in order to remove undesired signals (or cross talk) by adjacent tracks from the output radiation; an objective lens 106 for collimating the output radiation to produce an output data beam; a detector 108 for producing electrical signals in response to the output data beam incident thereupon; an actuator 101 for sliding the pickup module 100 in a radial direction relative to the holographic disk 200; and a lens actuator 107 for adjusting a vertical position of the objective lens 106 relative to the holographic disk 200. The actuator 101 and the lens actuator 107 are controlled by the control unit 300.
In a process of reproducing data stored in the holographic disk 200, the reference beam generated by the laser source 102 is reflected by the double-sided reflecting section 103 and the reflecting section 104 to reach the holographic disk 200 rotated at a predetermined speed by the spindle motor 210 at an incidence angle corresponding to that of a reference light beam which was used in a recording process of the data. The holographic disk 200 produces diffracted output radiation, which is filtered by the aperture plate 105 and then collimated by the objective lens 106 to make the output data beam. The output data beam is directed by the double-sided reflecting section 103 towards the detector 108, which converts the output data beam into the electrical signals. Then, the electrical signals are transmitted from the detector 108 to the signal processing unit 150.
Referring to FIG. 2, there are illustrated portions of tracks B and C of successive holograms stored in the holographic disk 200. The tracks B and C in the holographic disk 200 are spaced by a distance A, which is, for example, about 0.74 μm, and diameter of the laser beam illuminating the holographic disk 200 to reproduce the data stored therein is of the order of several tens of micrometers. Thus, the periphery portion of the laser beam illuminates the adjacent tracks C as well as the target track B, so that cross talk by the adjacent tracks C occurs during the reading process.
To prevent such a cross talk by filtering the diffracted output radiation, the aperture plate 105 having an opening of a desired diameter is disposed between the holographic disk 200 and the objective lens 106. However, such a holographic ROM system suffers from a drawback that the center of the opening of the aperture plate 105 should be aligned with the optical axis of the objective lens 106 to minimize distortion in reconstructed signals. The process of aligning the center of the opening of the aperture plate 105 with the optical axis of the objective lens requires high accuracy and is time-consuming, thereby increasing manufacturing cost of the holographic ROM system.